Where the North Sea meets the Baltic Sea1, there’s Grenen2; a spectacularly unspectacular sandbank at the northern end of Jutland, the continental part of Denmark. The form and position of the sandbank’s tip can vary considerably over short periods of time, when waves3 and currents remove or deposit sand. On the long term however, Grenen is growing by almost a kilometre (roughly half a mile) per century, slowly extending towards the north east. A closer look at the aerial picture shows distinct stripes in the landscape: successive layers of silt and sand that have accumulated over time. Of…